Electrical coupling system



Aug. 19, 1937. A KOLSTER 2,089,271

q ELECTRICAL COUPLING SYSTEM Original Filed Aug. 26, 1925 F'IIE-l- FIE E 33 R FIE- :3. u v 2 E I 32 35 v f c/ 13 33- INVENTUR A/Is nrousrs Patented Aug. 10, 1937 v PATENT OFFICE ELECTRICAL COUPLING SYSTEM Frederick A. Kolster, New York, N. Y., assignor to Federal Telegraph Company, San Francisco, CaliL, a corporation of California Application August 26. 1925, Serial No. 52,497

Renewed April 27, 1932 13 Claims. (01. 119-171) This invention relates to systems in which two electrical circuits are coupled together for transferring electrical energy from the one circuit to the other. Such systems are commonly used in coupling together the various high frequency circuits of radio receiving sets employing vacuum tube amplifiers.

It is anobject of this invention to devise a systern for coupling together two circuits so that the proportion of energy transferred from one circuit to the other will be a function of the frequency of the current, that is, as the frequency of the current increases the proportion of the energy transferred will decrease. It is proposed to apply such a system to selectively tuned radio frequency circuits. 7

More specifically it isan object of this invention to devise a coupling system in which the one circuit is provided with two branch parallel impedances one of which has a negative reactance value for all of the frequencies within the frequency range with which the system is adapted to be used, the total impedance of the two branches together having always a negative value of reactance within thesaid frequency range.

a novel system for coupling a circuit to the output of a vacuum tube by means of a reactance in the output circuit of the tube having a value n of reactance which is always negative thruout the frequency range with which the system is to be used and which reactance is shunted with a second reactance which in parallel with the first reactance will have a natural wave length below the minimum wave length of the frequency range. It is a further object of this invention to devise a coupling system for coupling together the out.-

put of the vacuum tube amplifier with another circuit in such a manner that there will always be 40 a substantially constant amplification ratio for the system and in which the proportion of the current transferred will always be relatively high.

Further objects of the invention will appear from the following description in which I have a set forth the preferred embodiment of my invention.

Referring to the figures of the'drawingr Figure 1 shows the coupling system of this invention applied to a vacuum tube circuit comprising tworadio frequency amplifiers.

Figure 2 is a curve illustrating the reactance values of thecoupling impedances.

Figure 3 is a modification of the system shown in Fig. 1. n In the past it has been common to couple together two electrical circuits by means of a coupling device comprising a primary and secondary inductance inductively coupled together and connected in the respective circuits. When this system is used in radio frequency circuits it cal It is afurther object of this invention to devise is the practice to employ a primary coil which is comparatively small so that its natural wave length will be below that of the wave length range with which-the system is adapted to be used. In this way local oscillations in the first circuit 'are prevented. However, this coupling device has several inherent disadvantages; the primary coil beingrelatively small, it cannot transfer a very large proportion of the current from the one circuit to the other and the system therefore has a very low efficiency. Also, since the reactance value of an inductance is directly proportional to the frequency of the current, the system will transfer a larger proportion of energy at high frequencies than at low frequencies. In the device of this invention these disadvantages have been overcome and an eflicient coupling device nas been produced which will not set up local oscillations in the first circuit and which will compensate for the increased reactance of the coupling inductance at the higher frequencies.

In the system of this invention there is preferably employed a primary and secondary inductance; but the primary inductance need not havea natural wave length period below that of the frequency range withwhich the system is to be used. It is proposed to operate the primary inductance in series with a capacity and to shunt both the capacity and the primary inductance with the second inductance. The values of the two inductances and the capacity are so selected that the reactance of the primary inductance together with the series capacity is always negative for a given frequency range. However, the value of the shunt inductance is such that the natural period of the capacity together with the two series inductances is always greater than the given wave length range of the system. Also, the combined reactive impedance of the system decreasesnegatively as a function of the frequency of the current.

In the drawing, the system has been shown as applied to the coupling of two radio frequency vacuum tube amplifiers, altho it is to be understood that the system is not limited to such use. Thus there has been shown two vacuum tube amplifiers ill and II having input circuits l2 and I3 respectively and output circuits i4 and I5 respectively. The amplifier I0 is preferably of the standard three element type and accordingly comprises a grid or control element ii, an anode or plate l1, and an electron-emission element or filament I8. The input circuit I? for the vacuum tube ll preferably includes an inductance I, one end of which is connected tothe grid I and the other end of which is connected to the terminal 20 of the filament ii. The input circuit i2 is coupled to a suitable source of signal energy such as an antenna 2| connected to the ground 22 thru a primary inductance 23 which is inductively coupled to the inductance I9. An A-battery 24 is provided to energize thefilament I8 in- A B-battery 25 is also prothe usual manner. vided to energize the output circuit I6 and has its negative terminal connected to the positive terminal of the battery 24. The positive terminal of the B-battery 25 is connected to the coupling device presently described. v

The vacuum tube II also comprises a grid 23, an anode or plate 21, and a filament 28. The input circuit I3 of the tube II preferably includes an inductance L3, one end of which is connected to the negative terminal of the A-battery 23. Means are also provided for selectively tuning the entire system over a, given frequency range. For this purpose there are preferably provided two variable condensers 32 and 33 which are shunted across the inductances I9 and L3 respectively. The A-battery 28 supplies energy to the filament 2812s .in the case of the tube II. The output circuit I5 includes the B-battery 30having its H negative terminal connectedto. the positive terw minal of the A-battery 29 and its positive ter-' minal connected to some suitable energy absorption device 3I which has been shown diagrammatically. I

In the output circuit l4 there, is provided a novel means for coupling together this output circuit to the input circuit I3 of the tube II. This coupling device preferably comprises two branch parallel impedances, one of which is coupled to-theinput circuit I3 These branchirnpedances preferably comprise an inductance L1 in series with a capacity C1, and an inductance L2 connected in parallel to L 01. The inductance L1 is preferably inductively coupled to the inproducethe curves illustrated in Fig. 2

Referring to Fig. 2, the abscissae represent d or two:- times the frequency of the current, while the ordinants represent values of reactances. The curve 34 represents a reactance curve of the inductance L1, while the. curve 35 represents the 4 reactance values of the inductance L with the series capacity C1. The branch La is theoretically apure inductance so that its curve 34 will be a straight line according to the following well known equation X2=wL2 Where:

vX1 is the reactance of branch L2. or is equal to 211- times the frequency. L1 is the absolute inductance of this branch.

From this expression it is seen that the rer t v For certain values of L101 there will be a value of'w at'which thereactance X1 will be zero. This corresponds to a' condition of resonance in the branch L'1C1 and is represented by the intersec tion ofthecurve 35 with the horizontal zero reactance line: 36 in accordance with the equation given below:

1 LIW=EI proximately 200 meters to 550 meters.

As 0; decreases from the point of intersection of the line 36 and the curve 35 the'reactance X1 will of course increase negatively and approach infinity. The values of L1C1 and L: are so selected that the lines 31 and 38 may represent the limit of the wave length range with which the system isadapte'd to be used. In the common broadcast receiver, this wave length range will be from ap- It should be noted that line 38 intersects the line 35 to the left of the intersection point between the line 35 and the line 38 andthat therefore the branch L1C1 does not become resonant within'this wave length range. it intersects the curve 34 at a greater distance from the'line 36 than the distance from the line 36 to the intersection of the line 31 with the curve 35. Thus at the value-of 0: represented by the Thus it will be seen that the values of L1C1 and Lrhave been so selected that within the range of frequencies intended to be covered by the system the reactance X1 of the branch L101 willalways be negative and will decrease as the frequency of the current increases and that the reactance of branch L2 always positive and increases as the frequency of the current increases.

The total series reactance of the two branches L'1C1 and L2 is represented by the curve 40 and is plotted in accordance with the following equation:

r t -e.)

Where: I V y X is the total reactance of the two branches 11101 and Lain series. 1

It'will be seen from this equation thatX will be resonant. when the conditions of Equation 4 aremet, that isywhere line 39 intersects line 36,

and will have the same value asbranch L2 at that value of a where branch L1C1 is resonant. Thus the total reactance of branchesL C1 and L2 in series will always bev positive and will increase as a function of the. frequency within the frequency range.

Thetotal reactance ofthe output circuit l4, that is,of the branches L1C1 and L2 in parallel, is shown by the curves ll of Fig. 2 and are plotted from the following equation: v

Wherez 1 X is the total output reactance of circuit l4. From theabove equation, itfollowsthat curve I will approach infinity on both sides'of the line 39,. andv that within the frequencyfrange represented by lines 31 and 38, will always be negative and will decrease negatively as the frequency increases. 1

In operation, there is an actual flow of local The line 31 is also so located that current between branches L1C1 and L2 because of the negative value of the reactance of branch L1C1. The more negative this branch becomes relative to the reactance of L1, the more local current will fiow. Thus as the frequency decreases within the frequency range, the current in the inductance L1 will increase and effect a greater transfer ofenergy to the inductance La, and the proportion of energy transferred from the circuit I4 to the circuit i3 will vary inversely as a function of the frequency. The output circuit M. will not oscillate locally because the natural wave length period of branches L1C1 and L2 in parallel is always below that of the wave length range covered bythe system. Furthermore the impedance circuit will not oscillate within itself because the natural period of Llcl and L2 in series is always greater than the longest wave length of the frequency range. Thevalue of inductance L1 may therefore be sufficiently large to effect efficient transfer of energy to the inductance L3 withoutcausing local oscillations.

In Fig. 3 there is shown a modification of the system described above and in this modification the inductances L1 and L2 are combined in a single coil 5| which is placed in inductive relation to the inductance La. ."Ihe capacity or condenser C1 is placed in series with the one terminal of the inductance 5i, while the other terminal of this inductance is connected to the anode of the vacuum tube 50 in the same manner as previously described. The B-battery 52 is connected to an intermediate tap onthe inductance 5| in order to divide this inductance into the two portions L1 and L2. Otherwise this modification is the same as that shownin Figure 1 and the operation is identical except that in this case both portions L1 and L2 are inductively coupled with the inductance Ls.

I claim: 7

1. In a radio amplifier stage including a vacuum tube having a filament, a grid and a plate,

an electric coupling system which comprises a main resonant circuit including a fixed secondary coil and an adjustable condenser'adaptedto tune the coupling system over a range in frequency, a fixed coil electromagnetically coupled to said secondary coil, a circuit including said fixed coil and a fixed capacity effectively in parallel, said fixed capacity being externalto said main resonant circuit, a primary coil electromagnetically coupled to said secondary coil, and a path through said coupling system between said plateand said filament including in series said fixed capacity and said primary coil, whereby in the operation of the amplifier stagethere is developed in said resonant circuit a resonant voltage whose ratio to'the voltage between the plate and the filament automatically rises whensaid adjustable con denser is adjusted for;higher frequencies.

2. In a radio amplifier stage including a vacuum tube having a filament, a grid and a plate, an electric coupling system which comprises a main resonant circuit including a fixed secondary coil and an adjustable condenser adapted to tune the coupling system over a range in frequency, a fixed coil electromagnetically coupled to said secondary coil, a circuit including said fixed coil and a fixed capacity effectively in parallel, and having a resonant frequency lower than the lowest frequency within said range, said fixed capacity being external to said main resonant circuit, a primary coil electromagnetically coupled to said secondary coil, and a path through said coupling system between said plate and said filament including in series said fixed capacity coupling system over a range in frequency, a fixed I coil electromagnetically coupled to said secondary coil, a circuit including said fixed coil and a fixed capacity effectively in parallel, said fixed capacity abeing external tosaid main resonant circuit, a primary coil electromagnetically coupled to said secondary coil, and a path through said coupling system between said plate and said filament including in series said fixed capacity and said primary coil, said primary coil having such polarity relative to said fixed coil that the terminal of the primary coil connected to one side of said fixed capacity is of opposite open-circuit polarity to the terminal of the fixed coil connected to the other side of the fixed capacity, whereby in the operation of the amplifier stage there is developed in said resonant circuit a resonant voltage whose ratio to the voltage between the plate and the filament automatically rises when said adjustable condenser is adjusted for higher frequencies.

4. In a radio amplifier stage including a vacuum tube having a filament, a grid and a plate, an electric coupling system which comprises a main resonant circuit including a fixed secondary coil and an adjustable condenser adapted to tune the coupling system over a range in frequency, a fixed coil electromagnetically coupled to said secondary coil, a circuit including said fixed coil and a fixed capacity effectively in parallel and having a resonant frequency lower than the lowest frequency within said range, said fixed capacity being external to said main resonant circuit, a primary coil electromagnetically coupled to said secondary coil, and a path through said coupling system between said plate and said filament including in series said fixed capacity and said primary coil, said primary coil having such polarity relative to said fixed coil that the voltages across said primary coil and said fixed capacity are additive, whereby in the operation of the amplifier stage there is developed in said resonant circuit a resonant voltage whose ratio to the voltage between the plate and the filament automatically rises when said adjustable condenser is adjusted for higher frequencies.

5. In a radio amplifier stage including a.

vacuum tube having a filament, a grid and a plate, an electric coupling system which comprises a main resonant circuit including as elements a coil and a condenser, at least oneof which is adjustable to tune the coupling system over arange in frequency, a second circuit coupled to said resonant circuit including a fixed self-inductance effectively in parallel with fixed capacityexternal to said main resonant circuit, said second circuit being resonant at a frequency lower, but not greatly lower, than the lowest frequency within said range, said fixed self-inductance being electromagnetically coupled to said resonant circuit, a circuit element having a substantially fixed voltage ratio relative to said main resonant circuit, and a path through said coupling system between said plate and said filament including in series said circuit element and at least a portion of said fixed capacity, whereby in the operation of the amplifier stage there is,

vacuum tube having a filament, a grid and a,

plate, an electric coupling system which comprises a main resonant circuit including a fixed secondary coil and an adjustable condenser adapted to tune the coupling system over a range in frequency, a fixed coil electromagnetically coupled to said secondary coil, a circuit including said fixed coil and a fixed capacity effectively in parallel, a primary coil electromagnetically coil-- pled to said secondary coil, and a path through said coupling system between said plate and said filament including in series said fixed capacity and said primary coil. l

7. In a radio amplifier stage including a vacuum tube having a filament, a grid and a plate, an electric coupling system which comprises a main resonant circuit including as elements a secondary coil and a condenser, at least one of which is adjustable to tune the coupling system over a range in frequency, a fixed coil electromagnetically coupled to said secondary coil, a circuit including said fixed coiland a fixed capacity efiectively in parallel, a primary coil main resonant circuit including as elements a' secondary coil and a condenser, at least one of which is adjustable to tune the coupling system over a range in frequency, a second circuit coupled to said resonant circuit including afixed self-inductance electromagnetically coupled tosaid secondary coil and effectively inparallel with fixed capacity, said second circuit being res-' onant at a frequency lowr than the-lowest fre quency within said range, a. primary coil elec-v trornagnetically coupled to saidsecondary coil,

and a path through said coupling system between" said plate and said filament including in series said primary coil and at least a portion of said fixed capacity. 7 a a 9. In a radio amplifier stage including a vacuum tube having a filament, a grid anda plate, an electric coupling system which comprises a main resonant circuit including as elements a coil and a condenser, at least one of which is adjustable to tune thecoiipling system over a range in frequency, a second circuit coupled to said resonant circuit including a fixed self-inductance efie'ctively'in parallel with fixed capacity external to said main resonant circuit, and being resonant at a frequency lower, 'but not greatly lower, than the lowest frequency within said range, said fixed self-inductance being electromagnetically coupled to said resonant circuit, a circuit element having a substantially fixed voltage ratio relative to said main resonant circuit, and apath through said coupling'system between said plate and said filament in- 5 eluding in series said circuit element. and at least a portion of said. fixed cap acity,-w,hereby in the operation of the amplifier stage there is developed in saidmain. resonant circuit a resonant voltage whose ratio to, the voltage be 10 tween the plateand the filament automatically .rises when said adjustable elements is adjusted for higher frequencies.

10. In a radio amplifier stage including a vacuum tube having a filament, a grid and a plate, 15 an electric coupling system which comprises a main resonant circuit including as elements a secondarycoil and a condenser, at least one of a which is adjustable to tune the coupling system over a range in frequency,. a second circuit 20 coupled to said resonant circuit including-a fixed self-inductance electromagnetically coupled to said secondary coil and effectively in parallel with fixed capacity, and being resonant at a frequency lower than the lowest frequency with- 5 in said range, a primary coil electromagnetically coupled to said secondary coil, and a path through said coupling system between said plate and said filament including .in series said primary coil and at least a portion of said fixed 30 capacity. I l r 11. In a radio frequency amplifier, a thermionic device having an output circuit comprising a plurality of inductors, a thermionic device having a tunable input circuit coupled to said 35 inductors, and means in effective shunt to one of said inductors to constitute: therewith, a circuit tuned to a frequency lower thanany frequency to which said input circuit is intended to ,be tunable.

12. In a radio frequency amplifier, a thermionic device having an output circuit comprising a plurality of inductors, a thermionic device having a tunable input circuit coupled tosaid inductors, and means in effective shunt to one 4 of said inductors to constitute therewith a circuit' tuned'to a frequency lower than any frequency to which said input circuit is intended to be tunable, and'fthe impedances of. said inductors andsaidmeans being so related that for 50 all frequencies to which saidinput circuit. is intended to be tunable said output circuit is negatively reactive.

13. In a radio frequency amplifier, a thermionic device having an output circuit comprising 55 a plurality of inductors, a thermionic device having a tunable input circuit coupled to said inductors, and means in circuit with one of said 65 FREDERICK a KOLs'rER. 

